1. Field of the Invention
The present invention relates to a polishing apparatus and a polishing method, and more particularly to a polishing apparatus and a polishing method for polishing and flattening a workpiece or substrate, such as a semiconductor wafer.
2. Description of the Related Art
With the recent progress toward higher integration of semiconductor devices, circuit interconnects are becoming finer and the distance between adjacent interconnects is becoming smaller. Especially when forming a circuit pattern by optical lithography with a line width of not more than 0.5 μm, a stepper requires a high flatness of imaging surface because of the small depth of focus. A polishing apparatus for carrying out chemical mechanical polishing or CMP, is known as a means for flattening a surface of such a semiconductor wafer.
As shown in FIG. 1, conventionally, a polishing apparatus comprises a turntable 504 having, on its upper surface, a polishing pad 502 having a polishing surface 500, a top ring 506 as a holding device for holding, on its lower surface, a semiconductor wafer W as a workpiece, and a liquid supply nozzle 508 as a liquid supply device for supplying a liquid Q, such as a slurry or a dressing liquid, to the polishing surface 500. When polishing a semiconductor wafer W by such a polishing apparatus, the semiconductor wafer W, which is held on the lower surface of the top ring 506, i.e., a holding device or a polishing head, is pressed against the polishing surface 500 at a predetermined pressure while supplying a slurry from the liquid supply nozzle 508 to the polishing surface 500 and moving the turntable 504 and the top ring 506 relative to each other. The semiconductor wafer W is thus kept rubbed with the polishing surface 500 in the presence of the slurry, whereby the surface of the semiconductor wafer W is polished into a flat mirror surface.
After completion of the polishing of the surface of the semiconductor wafer W, a so-called lift-off operation, which involves again attracting the semiconductor wafer W to the top ring 500, and lifting up the top ring 506 to detach and lift the semiconductor wafer W from the polishing surface 500 of the polishing pad 502, is carried out. At the start of the lift-off operation, a liquid Q, such as the slurry, a cleaning liquid or pure water, is present between the polishing pad 502 and the semiconductor wafer W. Due to the presence of the liquid Q, an attraction force is produced between the polishing pad 502 and the semiconductor wafer W. Therefore, in order to detach the semiconductor wafer W from the polishing surface 500 upon the lift-off operation, it is necessary to lift up the semiconductor wafer W with a force that counteracts the attraction force.
It is therefore a common practice to laterally move the top ring 506 holding the semiconductor wafer W and make the semiconductor wafer W overhang the turntable 504 by about ⅓ of the diameter so as to reduce the attraction force between the semiconductor wafer W and the polishing pad 502, as shown in FIG. 2, and then lift up the top ring 506 to detach the semiconductor wafer W from the polishing surface 500. A stable lift-off operation becomes possible by thus making the semiconductor wafer W overhang the polishing pad 502 and thereby reducing the attraction force between them. However, in such a lift-off operation which involves overhanging of the semiconductor wafer W, contact of an edge of the polishing pad 502 with the semiconductor wafer W can scratch the surface of the semiconductor wafer W.
On the other hand, when lifting a semiconductor wafer off a polishing pad after polishing without overhanging of the semiconductor wafer, there are cases where because of the strong attraction force acting between the polishing pad and the semiconductor wafer, the semiconductor wafer cannot be picked up, or the semiconductor wafer breaks due to a heavy load applied on it.